1. Field of the Invention
The present invention relates to an ion conductor structural body, a process for producing the ion conductor structural body, a rechargeable battery provided with the ion conductor structural body, and a process for producing the rechargeable battery. More particularly, the present invention relates to an ion conductor structural body having a high ion conductivity, a process for producing the ion conductor structural body, a rechargeable battery provided with the ion conductor structural body and which exhibits an improved charge-and-discharge efficiency, and a process for producing the rechargeable battery.
2. Prior Art
In recent years, the global warming of the earth because of the so-called greenhouse effect to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting to newly establish a thermal electric power plant. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Now, for electric vehicles which do not exhaust any air-polluting substances such as CO2, NOx, hydrocarbons and the like, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein. Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and cellular phones.
Since the use of a lithium-graphite intercalation compound as an anode of a rechargeable battery has been reported in JOURNAL OF THE ELECTROCHEMICAL SOCIETY vol. 117, No.2, p. 222-224 (1970), as such miniature, lightweight and high performance rechargeable battery, development has proceeded of rocking chair type lithium ion batteries (which are so-called lithium-ion batteries) in which, for instance, a carbonous material such as graphite is used as an anode active material and an intercalation compound incorporating a lithium ion is used as a cathode active material, wherein lithium is stored in the carbonous material as the anode active material by intercalating the lithium at intercalation sites in the carbonous material in the battery reaction upon charging. Some of these lithium-ion batteries have been put to practical use. In these lithium-ion batteries, by using the carbonous material as a host material capable of intercalating lithium as a guest material at the intercalation sites as the anode, occurrence or growth of a lithium dendrite upon charging is prevented to achieve a desired charge-and-discharge cycle life.
However, in such rechargeable battery in which the battery reaction (the charge-and-discharge reaction) by virtue of lithium ion is used, an organic solvent is usually used as the solvent of the electrolyte solution, wherein when the rechargeable battery is over-charged, there is a tendency in that the organic solvent is decomposed to produce carbon dioxide, hydrocarbons, and the like which are never recombined into the original solvent. Because of this, there is a fear that the electrolyte solution is deteriorated to increase the internal impedance of the rechargeable battery. Besides, when the rechargeable battery is over-charged, there will be an occasion in that internal-shorts between the anode and the cathode are occurred to generate heat and along the heat generation, decomposition of the electrolyte solution is rapidly progressed, resulting in a reduction in the performance of the rechargeable battery.
In order to solve the problems relating to the decomposition or the deterioration of the electrolyte solution of the rechargeable battery in which the charge-and-discharge reaction by virtue of lithium ion is used, U.S. Pat. No. 5,609,974 (hereinafter referred to as Document 1) proposes use of an ion conductor structural body obtained by copolymerizing a diacrylate series monomer, a monoacrylate series monomer and a carbonate group-bearing acrylate series monomer in the presence of an organic solvent and a retaining electrolyte. Besides, Japanese Unexamined Patent Publication No. 25353/1993 (hereinafter referred to as Document 2) proposes use of an ion conductor structural body in which a polymer matrix obtained by copolymerizing a diacrylate series monomer, a monoacrylate series monomer and a vinylene carbonate is used, aiming at preventing leakage of the electrolyte solution of the rechargeable battery. However, any of the ion conductor structural bodies disclosed in Document 1 and has a problem such that the ion conductivity thereof is less than ¼ of that of the electrolyte solution and therefore, when used in the rechargeable battery, it is difficult to achieve a desirably high energy density for the rechargeable battery. Separately, the results of experimental studies by the inventors of the present invention have revealed a fact that it is difficult to obtain an ion conductor structural body which has a necessary strength upon the production or use of a rechargeable battery in accordance with the technique described in Document 1 or 2 and the ion conductor structural body has an ion conductivity which is greatly decreased at low temperature which is lower than ordinary temperature, resulting in a sudden reduction in the energy density of the rechargeable battery.
Japanese Unexamined Patent Publication No. 95403/1995 (hereinafter referred to as Document 3) discloses an ion conductor structural body comprising a polymer matrix which is two-dimensionally cross-linked using a lipid, which is used in the rechargeable battery. Japanese Unexamined Patent Publication No. 224105/1995 (hereinafter referred to as Document 4) discloses an ion conductor structural body having a double continuous structure in which a hydrophilic polymer phase and a hydrophobic polymer phase are continued using a surface active agent, which is used in the rechargeable battery. However, in any of the techniques disclosed in Documents 3 and 4, there is a disadvantage in such that the lipid or the surface active agent is difficult to be completely removed in the washing process upon the preparation of the ion conductor structural body and the residual lipid or surface active agent results in deteriorating the charge-and-discharge cycle life of the rechargeable battery. There is also a disadvantage such that because the lipid or the surface active agent that is not connected with the polymer matrix is contained as described above, it is difficult to achieve a necessary mechanical strength for the ion conductor structural body upon the preparation thereof. Further, there is a disadvantage such that the removal of the lipid or the surface active agent in the washing process forms air gaps in the structure, resulting in a reduction in the mechanical strength of the ion conductor structural body.
In order to improve such shortcomings relating to the mechanical strength of the ion conductor structural body, Japanese Unexamined Patent Publication No. 299119/1993 (hereinafter referred to as Document 5) discloses an ion conductor structural body comprising a polymer phase having a high polarity and a polymer phase having a low polarity as a retaining phase. However, the ion conductor structural body disclosed in Document 5 has a disadvantage such that the low polarity polymer phase does not function as an ion conductor phase and because of this, the ion conductivity is insufficient. Besides, Japanese Patent Publication No. 3045120 (hereinafter referred to as Document 6) discloses an ion conductor structural body in which an alkylene oxide derivative having a substituent group comprising a liquid crystalline compound is used. Japanese Unexamined Patent Publication No. 303905/1993 (hereinafter referred to as Document 7) discloses an ion conductor structural body in which a polymer matrix obtained by curing a polyether group-bearing monomer is used. However, any one of the ion conductor structural bodies disclosed in Documents 6 and 7 has a disadvantage such that because the polymer matrix structure is irregular, the ion dispersing property is inferior and the ion conductivity is insufficient. Further, Japanese Unexamined Patent Publication No. 324114/1997 (hereinafter referred to as Document 8) and Japanese Unexamined Patent Publication No. 106345/1998 (hereinafter referred to as Document 9) disclose ion conductor structural bodies comprising a polymer matrix in which a polymer whose glass transition temperature is low is used. However, these ion conductor structural bodies disclosed in Documents 8 and 9 have disadvantages as described in the following. That is, the polymer chain of the polymer matrix is likely to be softened and because of this, the mechanical strength of the ion conductor structural body is insufficient. In Documents 8 and 9, it is described that a plasticizer is not necessary to be used. However, in accordance with the techniques described in Documents 8 and 9, except for using a plasticizer, the present inventors prepared a gel-like ion conductor structural body comprising a polymer matrix in which a plasticizer is used, and an evaluation was conducted. As a result, it was found that the softening temperature of the polymer matrix is lowered due to the plasticizer. In addition, the rechargeable battery in which such gel ion conductor structural body is used was found to have a disadvantage in that when the battery temperature is increased, the anode and the cathode begin to suffer from internal shorts between them.
Separately, Japanese Unexamined Patent Publication No. 302410/1999 (hereinafter referred to as Document 10), Japanese Unexamined Patent Publication No. 2000-119420 (hereinafter referred to as Document 11), and Japanese Unexamined Patent Publication No. 2000-212305 (hereinafter referred to as Document 12) disclose orientation type ion exchange films comprising a specific monomer structure. Any of the orientation type ion exchange films disclosed in Documents 10 to 12 provides effects as desired unless a plasticizer is contained therein. However, any of the techniques disclosed in Documents 10 to 12 is insufficient in order to form a polymer matrix structure having a regularity suitable for use in an ion conductor structural body in which use of a plasticizer such as a solvent is essential. Thus, it is difficult to achieve an ion conductor structural body having a high ion conductivity.
Incidentally, Japanese Unexamined Patent Publication No. 345629/1999 discloses a rechargeable battery having a prolonged charge-and-discharge cycle life in which an ion channel is orientated. In order to more improve the characteristics of the rechargeable battery, it is necessary to provide an ion conductor structural body having a high ion conductivity and an excellent mechanical strength.